Go here:
https://everquest.allakhazam.com/user.html?1 Click on "your files" and upload the avatar image you wish to use. It must be a gif or jpg, and 120 x 120 pixles in diameter.
Once your file is uploaded, go back to
http://ffxi.allakhazam.com/user.html?1
Then you should have an avatar image to select from the drop down in the lower half of that page.